Process for preparing polymers from vinylaromatic compounds by dispersion polymerization

ABSTRACT

A process for preparing polymers of vinylaromatic compounds in dispersion comprises conducting the polymerization in the presence of a dispersing auxiliary and a catalyst obtainable from A) a transition metal complex of subgroups II to VIII, B) a cation-forming agent and C), if desired, an aluminum compound.

The invention relates to a process for preparing polymers ofvinylaromatic compounds in dispersion in the presence of metallocenecatalyst systems.

The polymers thereby obtainable can be used to produce fibers, films andmoldings.

Polymerizing styrene in the presence of metallocene catalyst systemsleads to polymers of high stereoregularity and is described at length,for example, in EP-A 0 210 615. Because of its high crystallinity,syndiotactic polystyrene has a very high melting point of about 270° C.,high rigidity and tensile strength, dimensional stability, a lowdielectric constant and high chemical stability. The profile ofmechanical properties is retained even above the glass transitiontemperature.

In the metallocene-catalyzed polymerization of styrene, there isfrequently crystallization of the resulting syndiotactic polystyrenestarting at a level of only about 10% conversion. This leads firstly tothe formation of deposits on the walls and secondly to an extremeviscosity rise during the polymerization, which makes handling anddissipation of the heat of reaction more difficult, especially on theindustrial scale.

To solve this problem a variety of techniques using special reactors orextruders have been tried out. EP-A-0 535 582 describes a process forpreparing syndiotactic polystyrene in a stirred bed of solids, which isable to reduce the wall deposits but not prevent them. The reactor hasto be equipped with a special stirrer in order to produce a homogeneousfluidized bed. Temperature control is by way of partial evaporation ofstyrene by reduced pressure, using a complex vacuum control system.

EP-A 0 584 646 and EP-A 0 389 939 describe the preparation ofsyndiotactic polystyrene in self-cleaning twin-screw extruders orcompounders with no dead spaces. In both processes, owing to the suddenrise in frictional forces at higher levels of conversion, and to themotor output required for continued operation, polymerization is carriedout not to complete conversion but only to a level where the polymerpowder, soaked with monomers, no longer agglomerates in the course ofsubsequent processing steps.

In the case of anionic initiation, the technique of dispersionpolymerization is known. It is employed specifically to prepare smallpolystyrene particles, as described for example in Journal of PolymerScience, Part A, Polymer Chemistry, Vol. 34 (1996), pages 2633-2649. Ofcritical importance is the selection of the dispersing auxiliary forstabilizing the dispersion.

DE-A 43 30 969 describes a process for preparing polystyrene mixtures bypolymerizing styrene in an organic liquid in the presence of astyrene-butadiene block copolymer and of a metallocene catalyst system.For the preferred embodiment, however, pressures of from 5 to 20 bar arerequired; otherwise the resulting polymers have a very low molecularweight of around 30,000 g/mol.

It is an object of the present invention to provide a process forpreparing syndiotactic vinylaromatic polymers using metallocenecatalysts, which does not have the above disadvantages and can becarried out in customary stirred reactors at low viscosities.

We have found that this object is achieved by conducting themetallocene-catalyzed polymerization of vinylaromatic monomers indispersion using styrene/diphenylethylene-diene block copolymers asdispersing auxiliaries.

Particularly suitable vinylaromatic compounds are those of the formula I

where

R¹ is hydrogen or C₁-C₄-alkyl,

R² to R⁶ independently are hydrogen, C₁-C₁₂-alkyl, C₆-Cl₈-aryl orhalogen, or two adjacent radicals together are cyclic groups having 4 to15 carbons, for example C₄-C₈-cyclo-alkyl, or fused ring systems.

It is preferred to employ vinylaromatic compounds of the formula I inwhich

R¹ is hydrogen.

Particularly suitable substituents R² to R⁶ are hydrogen, C₁-C₄-alkyl,chlorine or phenyl, biphenyl, naphthalene or anthracene. Two adjacentradicals may also together be cyclic groups having 4 to 12 carbons, sothat compounds of the formula I may also, for example, be naphthalenederivatives or anthracene derivatives.

Examples of such preferred compounds are:

styrene, p-methylstyrene, p-chlorostyrene, 2,4-dimethylstyrene,4-vinylbiphenyl, 2-vinylnaphthalene or 9-vinylanthracene.

It is also possible to employ mixtures of different vinylaromaticcompounds, in which case one component may also carry furtherhydrocarbon radicals, such as vinyl, allyl, methallyl, butenyl orpentenyl groups, preferably vinyl groups, on the phenyl ring. It ispreferred, however, to use only one vinylaromatic compound.

Particularly preferred vinylaromatic compounds are styrene andp-methylstyrene.

The preparation of vinylaromatic compounds of the formula I is known perse and is described, for example, in Beilstein 5, 367, 474, 485.

Suitable dispersion auxiliaries are block copolymers having at least onediene block B and at least one block S comprising a copolymer of avinylaromatic monomer of the formula (I) and 1,1-diphenylethylene or itsaromatic ring-substituted derivatives, including those substituted withalkyl of up to 22 carbons, as are described, for example, in DE-A 44 20917.

Suitable examples are block copolymers with blocks S and B, of thegeneral structures (S—B)_(n), S—B—S, B—S—B, X[(S—B)_(n)]_(m),X[(BS)_(n)]_(m), X(S—B—S)_(m) and X(B—S—B)_(m), where X is the radicalof an m-functional coupling agent or of an m-functional initiator, n isan integer from 1 to 5 and m is an integer from 2 to 20.

All dienes are suitable in principle as the diene component for theblock B, although preference is given to those having conjugated doublebonds, such as butadiene, isoprene, dimethylbutadiene andphenylbutadiene. The diene block may be partially or completelyhydrogenated or unhydrogenated. The molecular weights Mw of the blocks Bare generally from 10,000 to 500,000, preferably from 50,000 to 350,000and, with particular preference, from 70,000 to 250,000, g/mol.

The blocks S consist of a copolymer of a vinylaromatic monomer of theformula (I) and 1,1-diphenylethylene or its ring-substitutedderivatives, including those substituted with alkyl of up to 22 carbons,preferably of 1 to 4 carbons, such as methyl, ethyl, isopropyl, n-propyland n-, iso- or tert-butyl. Particular preference, however, is given tothe use of unsubstituted 1,1-diphenylethylene itself. The proportion ofdiphenylethylene in the block S is from 15 to 65% by weight, preferablyfrom 25 to 60% by weight. The molar ratio of the units derived from thevinylaromatic monomer to units derived from 1,1-diphenylethylene isgenerally in the range from 1:1 to 1:25, preferably from 1:1.05 to 1:15and, with particular preference, in the range from 1:1.1 to 1:10.

The copolymer block S is preferably random in composition and has amolecular weight Mw of in general from 20,000 to 500,000, preferablyfrom 50,000 to 300,000. Particular preference is given to a copolymerblock S of styrene and 1,1-diphenylethylene.

The block ratio S to B is generally in the range from 90:10 to 20:80,particularly preferably from 90:15 to 65:35. The block transitions canbe either clean-cut or tapered. A tapered transition is one where theadjacent blocks B and S may, in the transition region, also containmonomers of the other block.

The block copolymers can be prepared by customary methods of anionicpolymerization, as described for-example in M. Morton, AnionicPolymerisation, Principles and Practice, Academic Press, New York 1983.The anionic polymerization is initiated by means of organometalliccompounds. Preference is given to compounds of the alkali metals,especially of lithium. Examples of initiators are lithium alkyls such asmethyllithium, ethyllithium, isopropyllithium, n-, sec- ortert-butyllithium. It is particularly preferred to employ n- ors-butyllithium. Suitable solvents are those which are inert toward theorganometallic initiator. Aliphatic or aromatic hydrocarbons arejudiciously used. Examples of suitable solvents are cyclohexane,methylcyclohexane, benzene, toluene, ethylbenzene and xylene.

To influence the polymerization parameters, small amounts of polaraprotic substances may be added to the solvent. Suitable examples areethers, such as diethyl ether, diisopropyl ether, diethylene glycoldimethyl ether, diethylene glycol dibutyl ether or, in particular,tetrahydrofuran, and also tertiary amines, such astetramethylethylenediamine or pyridine. The polar cosolvent is added tothe apolar solvent in a small amount of from about 0.01 to 5% by volume.Particular preference is given to tetrahydrofuran in an amount of fromabout 0.1 to 0.3% by volume.

In a preferred embodiment of the novel process, at least one branchingmonomer can be employed.

As branching monomers it is possible to use compounds of the formula II

where

R^(a) is hydrogen, halogen or an inert organic radical of up to 20carbons, where if p≧2 each R^(a) can be identical or different and tworadicals R^(a) can form a 3- to 8-membered ring together with the metalatom attached to them, and R^(a) can also be a customary complex ligandif M is a transition metal,

R^(b) is hydrogen, C₁-C₄-alkyl or phenyl;

R^(c) is hydrogen, C₁-C₄-alkyl, phenyl, chlorine or an unsaturatedhydrocarbon radical of 2 to 6 carbons;

M is C, Si, Ge, Sn, B, Al, Ga, N, P, Sb, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,W, Mn, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Zn or Cd,

n is 2-6;

m is 0-20;

p is 0-4;

with the proviso that the sum of n+p corresponds to the valency of M.

These monomers can be obtained, for example, by way of the Grignardcompounds of the chloro(alkyl)styrenes with the corresponding carbon,metal or transition metal compounds, for example the halogen compounds.Where M is silicon, germanium or tin, for example, such reactions aredescribed in K. Nakanishi, J. Chem. Soc. Perkin Trans I, 1990, page3362.

Particularly preferred branching monomer units are those of the formulaII in which M is carbon, silicon, germanium, tin or titanium, becausethey are easy to obtain. The index m is preferably from 0 to 8,particularly preferably from 0 to 4.

For example, the titanium-containing monomers of the formula IIa

and the titanium compound IIb

where R^(a), R^(b), R^(c), m, n and p are as defined above, can beemployed as branching monomers.

The inert organic radical or radicals R^(a) are not of great importanceto the process. Rather, they serve merely to satisfy the free valenciesof M and can be selected for ease of availability. Examples of suitableradicals are aliphatic and cycloaliphatic radicals, aryls, hetaryls andaralkyls. Aliphatic radicals include alkyls, alkoxys, alkenyls oralkynyls having, for example, from 1 to 2 or 20 carbons. Cycloaliphaticradicals include cycloalkyls or cycloalkane radicals of 3 to 8 carbons.Instead of a methylene in the alkyl or cycloalkyl it is also possiblefor there to be an oxygen in ether function. Examples of aryls arephenyls or naphthyls, it also being possible for two phenyls to beconnected by an oxygen. Examples of aralkyls are those of 7 to 20carbons that result from combination of a phenyl with an alkyl. Examplesof hetaryls are pyridyl, pyrimidyl and furyl. These radicals can also besubstituted further, for example by alkyl, alkoxy, halogen, such asfluorine, chlorine or bromine, cyano, nitro, epoxy, carbonyl, estergroups, amides, and so on. Two of the radicals R^(a) can also form a 3-to 6-membered ring with the atom M, for example where two radicals R^(a)form an alkylene chain in which one or more CH₂ groups may also havebeen replaced by 0 in ether function.

If M is a transition metal, R^(a) can also be a customary σ- or π-bonded complex ligand, such as ethylene, allyl, butadiene orcyclopentadiene, mono- or polysubstituted cyclopentadienes, such asmethylcyclopentadiene or pentamethylcyclopentadiene, benzene,cyclohexadiene, cycloheptatriene, cycloheptadiene, cyclooctatetraene,cyclococtatriene, cyclooctadiene, carbonyl, oxalato, cyano, isonitrile,fulminato-C, fulminato-O, cyanato, dinitrogen, ethylenediamine,diethylenetriamine, triethylenetetramine, ethylenediaminetetraacetate,nitrosyl, nitro, isocyano, pyridine, α,α-bipyridyl, trifluorophosphane,phosphane, diphosphane, arsane, acetylacetonato.

R^(b) is with particular preference hydrogen or methyl. R^(c) ishydrogen, C₁-C₄-alkyl such as methyl, ethyl, propyl, isopropyl, n-butyland the isomeric butyls, phenyl, chlorine or an unsaturated hydrocarbonradical of 2 to 6 carbons such as vinyl, allyl, methallyl, butenyl orpentenyl.

The branching monomer unit is judiciously employed in a molar ratio ofvinylaromatic monomer to branching unit of from 10,000,000:1 to 10:1.

In accordance with the invention, transition metal complexes ofsubgroups II to VIII, preferably III to VIII, are used as catalystcomponent A). Very particular preference is given to complexes of themetals titanium, zirconium and hafnium.

If the branching monomer unit of the formula II already has a transitionmetal M, especially titanium, then depending on the concentration usedit can also simultaneously be employed as catalyst component A inaddition to its function as a branching unit.

Particularly preferred catalyst components A) are metallocene complexes,especially those of the formula III

where

R⁷ to R¹¹ are hydrogen, C₁-C₁₀-alkyl, 5- to 7-membered cycloalkyl whichin turn can carry C₁-C₆-alkyls as substituents, C₆-C₁₅-aryl orarylalkyl, and where two adjacent radicals may if desired together becyclic groups of 4 to 15 carbons, for example fused ring systems is[sic] 4 to 12 carbons, or are Si(R¹²)₃,

where R¹² is C₁-C₁₀-alkyl, C₆-C₁₅-aryl or C₃-C₁₀-cycloalkyl,

M is a metal from subgroups III to VI of the Periodic Table of theElements or is a metal of the lanthanide series,

Z¹ to Z⁵ are hydrogen, halogen, C₁-C₁₀-alkyl, C₆-C₁₅-aryl, C₁-C₁₀-alkoxyor C₁-C₁₅-aryloxy

and

z₁ to z₅ are 0, 1, 2, 3, 4 or 5, the sum z₁+z₂+z₃+z₄+z₅ corresponding tothe valency of M minus 1.

Particularly preferred metallocene complexes of the formula III arethose in which

M is a metal from subgroup IV of the Periodic Table of the Elements,i.e. titanium, zirconium or hafnium, especially titanium,

and

Z¹ to Z⁵ are C₁-C₁₀-alkyl, C₁-C₁₀-alkoxy or halogen.

Examples of such preferred metallocene complexes are:

pentamethylcyclopentadienyltitanium trichloride,pentamethylcyclopentadienyltitanium trimethyl andpentamethylcyclopentadienyltitanium trimethylate.

It is also possible to employ those metallocene complexes described inEP-A 584 646.

Mixtures of different metallocene complexes can also be used.

Complex compounds of this kind can be synthesized by methods known perse, preference being given to reacting the correspondingly substituted,cyclic hydrogen anions with halides of titanium, zirconium, hafnium,vanadium, niobium or tantalum.

Examples of appropriate preparation techniques are described, interalia, in Journal of Organometallic Chemistry, 369 (1989), 359-370.

As compound B which forms cations, especially metallocenium ions, thecatalyst systems can comprise open-chain or cyclic alumoxane compounds.

Suitable examples are open-chain or cyclic alumoxane compounds of theformula IV or V

where R¹³ is C₁-C₄-alkyl, preferably methyl or ethyl, and k is aninteger from 5 to 30, preferably from 10 to 25.

The preparation of these oligomeric alumoxane compounds is usuallycarried out by reacting a solution of a trialkylaluminum with water andis described, inter alia, in EP-A 284 708 and U.S. Pat. No. 4,794,096.

In general, the oligomeric alumoxane compounds obtained are in the formof mixtures of both linear and cyclic chain molecules of differentlengths, so that k is to be regarded as an average value. The alumoxanesmay also be present in a mixture with other metal alkyls, preferablywith aluminum alkyls.

It has been found advantageous to use the metallocene complexes and theoligomeric alumoxane compound in amounts such that the atomic ratiobetween aluminum from the oligomeric alumoxane and the transition metalfrom the metallocene complexes is in the range from 10:1 to 10⁶:1, inparticular from 10:1 to 10⁴:1.

As compound B) forming metallocenium ions it is also possible to employcoordination complex compounds selected from the group consisting ofstrong, neutral Lewis acids, ionic compounds having Lewis-acid cationsand ionic compounds having Brönsted acids as cations.

Preferred strong neutral Lewis acids are compounds of the formula VI

M¹X¹X²X³  (VI)

where

M¹ is an element from main group III of the Periodic Table, especiallyB, Al or Ga, preferably B,

X¹,X² and X³ are hydrogen, C₁-C₁₀-alkyl, C₆-C₁₅-aryl, alkylaryl,arylalkyl, haloalkyl or haloaryl each of 1 to 10 carbons in the alkyland 6 to 20 carbons in the aryl, or are fluorine, chlorine, bromine oriodine, especially haloaryls, preferably pentafluorophenyl.

Particular preference is given to compounds of the formula VI in whichX¹, X² and X³ are identical; preferably tris(pentafluorophenyl)borane.These compounds and processes for their preparation are known per se andare described, for example, in WO93/3067.

Suitable ionic compounds having Lewis-acid cations are compounds of theformula VII

[(Y^(a+))Q₁Q₂ . . . Q_(z)]^(d+)  (VII)

where

Y is an element from main groups I to VI or subgroups I to VIII of thePeriodic Table,

Q₁ to Q_(z) are radicals with a single negative charge, such asC₁-C₂₈-alkyl, C₆-C₁₅-aryl, alkylaryl, arylalkyl, haloalkyl or haloaryleach having 6 to 20 carbons in the aryl and 1 to 28 carbons in thealkyl, C₁-C₁₀-cycloalkyl, which can be unsubstituted or substituted byC₁-C₁₀-alkyls, or are halogen, C₁-C₂₈-alkoxy, C₆-C₁₅-aryloxy, silyl ormercaptyl, such as trimethylsilyl,

a is an integer from 1 to 6,

z is an integer from 0 to 5, and

d corresponds to the difference a−z, but is greater than or equal to 1.

Particular suitability is possessed by carbonium cations, oxoniumcations and sulfonium cations, and also cationic transition metalcomplexes. Particular mention may be made of the triphenylmethyl, silverand 1,1′-dimethylferrocenyl cations.

They preferably have noncoordinating counterions, especially boroncompounds, as also mentioned in WO 91/09882, preferablytetrakis(pentafluorophenyl) borate.

Ionic compounds with Brönsted acids as cations and preferably also withlikewise noncoordinated counterions are specified in WO 93/3067; apreferred cation is N,N-dimethylanilinium.

It has been found to be particularly appropriate if the molar ratio ofboron from the compound that forms metallocenium ions to transitionmetal from the metal complex is in the range from 0.1:1 to 10:1, inparticular from 1:1 to 5:1.

The catalyst system employed in the novel process may include ascomponent C) an aluminum compound, for example of the formula VIII

AlR¹⁴R¹⁵R¹⁶  (VIII),

where

R¹⁴ to R¹⁶ are hydrogen, fluorine, chlorine, bromine, iodine orC₁-C₁₂-alkyl, preferably C₁-C₈-alkyl.

Preferably, R¹⁴ to R¹⁵ are identical and are C₁-C₆-alkyl, such asmethyl, ethyl, isobutyl or n-hexyl, and R¹⁶ is hydrogen.

The content of component C) in the catalyst system is preferably from1:2000 to 1:1, in particular from 1:800 to 1:10 (molar ratio oftransition metal from III to Al from VIII).

As solvents for the metallocene complexes it is common to employaromatic hydrocarbons, preferably those having 6 to 20 carbons, andespecially xylenes, toluene and ethylbenzene and mixtures thereof.

The metallocene complexes can be employed with or without a support.

Examples of suitable support materials are silica gels, preferably thoseof the formula SiO₂.bAl₂O₃, where b is a number from 0 to 2, preferablyfrom 0 to 0.5; i.e. essentially alumosilicates or silicon dioxide. Thesupports preferably have a particle diameter of from 1 to 200 μm, inparticular from 30 to 80 μm. Such products are obtainable commercially,for example as silica gel 332 from Grace.

Further supports include finely divided polyolefins, for example finelydivided polypropylene or polyethylene, and also polyethylene glycol,polybutylene terephthalate, polyethylene terephthalate, polyvinylalcohol, polystyrene, syndiotactic polystyrene, polybutadiene,polycarbonates and copolymers thereof.

The molar ratio of transition metal catalyst A) to vinylaromatic monomeris generally from 1:1000 to 1:10,000,000, but preferably from 1:2000 to1:1,000,000.

The process according to the invention is conducted as a dispersionpolymerization. The dispersing medium employed may judiciously comprisealiphatic hydrocarbons, especially those of 4 to 10 carbon atoms, orhydrocarbon mixtures. Examples are butane, pentane, hexane and heptane.The concentration of the monomers that are to be polymerized in thedispersion medium is in general from 5 to 65 percent by volume,preferably from 10 to 50% by volume.

The dispersing auxiliary is preferably used in an amount of from 0.1 to10% by weight, particularly preferably from 1 to 8% by weight, based onthe vinylaromatic compound employed. It is judiciously dissolved in thevinylaromatic monomer that is to be polymerized.

The polymerization conditions are not critical. Polymerization ispreferably conducted at from 50 to 100° C. under a pressure of from 0.05to 30 bar, preferably from 0.1 to 20 bar. The polymerization isgenerally at an end after from 0.5 to 10 hours. It can be terminated byadding protic compounds, for example methanol, and the dispersion mediumcan be removed by filtration or evaporation and recycled to the process.

The novel process is technically simple and permits the preparation ofvinylaromatic polymers having a high syndiotactic content with lowviscosities of less than 4 mPas in customary stirred vessels.Furthermore, the polymers are obtained in particulate form. Theresultant polymers are suitable for producing fibers, films andmoldings.

The preferred procedure in the novel process is to prepare a solution ofthe dispersing auxiliary in the vinylaromatic monomer, with aconcentration of from 0.1 to 10% by weight, preferably from 1 to 8% byweight, based on the vinylaromatic compound, and to heat the resultingsolution with, say, pentane to 30° C., for example. Then the calculatedamount of the catalyst components is added and polymerization isconducted at from 60 to 70° C. and is allowed to proceed to completion(about 1 hour) before being terminated with methanol.

The dispersion medium can be removed by filtration or by evaporation andthe solid obtained can be dried under reduced pressure. If desired, thepolymer can be purified by customary methods of plastics technology, forexample by reprecipitation or by washing with acids or alkalis.

EXAMPLES

Purifying 1,1-diphenylethylene (DPE)

Crude DPE (Aldrich or prepared by reacting phenylmagnesium bromide withacetophenone, acetylating with acetic anhydride and thermallyeliminating the acetic acid) is distilled to 99.8% purity on a columnhaving at least 50 theoretical plates (spinning band column; for largerquantities, a column with Sulzer packing). The distillate, which isusually pale yellow, is filtered through a 20 cm alox column (Woelmalumina for chromatography, anhydrous), titrated with 1.5 Nsec-butyllithium until there is a strong red coloration, and distilledover under reduced pressure (1 mbar). The resulting product iscompletely colorless and can be employed directly in the anionicpolymerization.

Purifying the monomers and solvent

The cyclohexane (H) employed as solvent was dried over anhydrous aluminaand titrated with the adduct of sec-butyllithium and1,1-diphenylethylene until a yellow coloration was obtained. Thebutadiene (Bu) was distilled off from triisobutyaluminum, the1,1-diphenylethylene (DPE) from sec-butyllithium (s-BuLi). A 0.5 molarsolution of s-BuLi in cyclohexane was used as initiator. Styrene (S) wasdried over alumina directly before use.

All polymerizations were conducted under purified nitrogen with rigorousexclusion of air and moisture. The reactors were pretreated for a numberof hours with a solution of 1,1-diphenylethylene and sec-butyllithium incyclohexane under reflux before being filled.

In the Examples below, Bu is 1,3-butadiene, S is styrene and DPE is1,1-diphenylethylene. Also, the proportions are by weight.

Preparing Bu-S/DPE block copolymers

Dispersant D1

7.1 l of cyclohexane and a few drops (about 2 ml) of DPE were charged toa 10 l stirred reactor and titrated with a 0.278 molar sec-butyllithiumsolution until the mixture began to take on a red coloration. Followingthe addition of 15.1 ml (4.2 mmol) of the 0.278 molar sec-butyllithiumsolution, 1.6 l (19.4 mol)) of 1,3-butadiene were added in portions (100ml) over the course of one hour at 70° C. and the mixture waspolymerized at 70° C. for a further hour. The molecular weights of theresulting polybutadiene block were determined on a sample by means ofgel permeation chromatography (GPC) with polybutadiene calibration:M_(w)=248,000 g/mol, M_(w)/M_(n)=1.28, M (peak maximum)=226,000 g/mol.To the resulting polybutadiene block there were added, in succession atan interval of 15 minutes, 98.3 ml (0.56 mol) of 1,1-diphenylethyleneand 259 ml (2.25 mol) of styrene, and polymerization was continued at70° C. for 5 hours more. After the reaction had subsided, the reactionmixture was titrated with ethanol until it became colorless and wasacidified with CO₂/water. The colorless solution was freed from solventunder reduced pressure in a devolatilizing extruder, and the product wasgranulated.

GPC (polybutadiene calibration): two peaks: 1st peak (20%) M (peakmaximum)=32,000 g/mol; 2nd peak (80%): peak maximum at 260,000 g/mol.

Dispersant D2

Following the procedure used for dispersant D1, 1.6 l of 1,3-butadiene,98.3 ml of 1,1-diphenylethylene and 259 ml (2.25 mol) of styrene werepolymerized, the polymerization being initiated with 10 ml (2.78 mmol)of a 0.278 molar sec-butyllithium solution.

GPC polybutadiene block (polybutadiene calibration): M_(w)=441,000g/mol, M_(w)/M_(n) =1.25, M (peak maximum)=352,000 g/mol

GPC block copolymer (polybutdiene calibration): two peaks, 1st peak(20%) M (peak maximum)=61,000 g/mol; 2nd peak (80%): peak maximum at411,000 g/mol.

Dispersant D3

Following the procedure used for dispersant D1, 1.2 l (14.5 mol) of1,3-butadiene, 73.6 ml (0.42 mol) of 1,1-diphenylethylene and 194 ml(1.69 mol) of styrene were polymerized, the polymerization beinginitiated with 7.5 ml (2.08 mmol) of a 0.278 molar sec-butyllithiumsolution.

GPC polybutadiene block (polybutadiene calibration): M_(w)=379,000g/mol, M_(w)/M_(n) =1.30, M (peak maximum)=324,000 g/mol

GPC block copolymer (polybutadiene calibration): two peaks, 1st peak(30%) M (peak maximum)=57,000 g/mol; 2nd peak (70%): peak maximum at394,000 g/mol.

Preparing an S/DPE-Bu-S/DPE triblock copolymer

Dispersant D4

Following the procedure used for dispersant D1, 1.08 l (13.1 mol) of1,3-butadiene, 149.5 ml (0.85 mol) of 1,1-diphenylethylene and 252.4 ml(2.2 mol) of styrene were polymerized, the polymerization beinginitiated with 49.1 ml (13.7 mmol) of a 0.278 molar sec-butyllithiumsolution. The polymerization was not terminated with ethanol, butinstead a solution of 0.5 g of ethyl formate (coupling agent) in 10 mlof cyclohexane was added over the course of 5 minutes. Working up was asdescribed for dispersant 1.

GPC block copolymer (mixed calibration for polystyrene and polybutadiene40:60): two peaks: 1st peak (10%) M (peak maximum) =79,000 g/mol; 2ndpeak (90%): peak maximum at 160,000 g/mol.

Example 1

250 ml of n-hexane and a mixture of 2.61 g of the dispersant D1 in 104.2g (1 mol) of styrene were introduced with stirring into a 2 l stirredreactor and heated to 60° C. 8.16 ml of a 1.53 molar solution ofmethylaluminoxane (MAO) in toluene (obtained from Witco), 2.08 ml of a1.0 molar solution of diisobutylaluminum hydride (DIBAH) in cyclohexane(obtained from Aldrich) and 9.5 mg (0.04 mmol) ofpentamethylcyclopentadienyltitanium trimethyl Cp*TiMe₃ were added. After5 minutes, a milky dispersion had formed. The viscosity of thedispersion remained below 1.4 mPas throughout the reaction period. After2 hours, the polymerization was terminated by adding 10 ml of methanoland the reaction mixture was filtered. The filter residue was washedwith methanol and dried at 50° C. under reduced pressure.

The amount of dispersant was 2.5% by weight based on the amount ofstyrene employed. The conversion, based on the amount of styreneemployed, was 33%. The molecular weight Mw and the molecular weightdistribution were determined by high-temperature gel permeationchromatography GPC (135° C., 1,2,4-trichlorbenzene, polystyrenestandard) as M_(w)=201,200, Mw/Mn=1.9. The syndiotactic content asdetermined by ¹³C—NMR spectroscopy was 96%. The particle size was in therange from 2 to 10 μm and was determined under the transmissionmicroscope (Axiophot from Carl Zeiss) on a sample of the polymersuspended in immersion oil and placed between two planar glass slides.

Examples 2 to 5

Example 1 was repeated with in each case 2.5% by weight of dispersantsD2, D3 and D4 or a 1:1 mixture of the dispersants D1 and D2, based ineach case on the amount of styrene employed. The results are summarizedin Table 1.

Comparison Experiment V1

Example 1 was repeated without the use of a dispersant. The resultingpolymer precipitated after only 10 minutes. The conversion, based on thestyrene employed, was only 16%.

Viscos- Conver- Syndiotac- M_(w) ity sion ticity Example Dispersant[g/mol] M_(w)/M_(n) [mPas] [%] [%] 1 D1 201,200 1.9 1.34 33 98 2 D2467,400 2.5 2.11 63 95 3 D3 367,800 2.3 1.96 59 97 4 D1 + D2 441,000 2.72.20 72 98 (each 1.25% by wt.) 5 D4 501,300 2.6 2.31 86 96 V1 — 248,2002.1 16 96

We claim:
 1. A process for preparing polymers of vinylaromatic compounds in dispersion in the presence of a dispersing auxiliary and a catalyst obtained from A) a transition metal complex of subgroups II to VII, B) a cation-forming agent and C), optionally, an aluminum compound, wherein the dispersing auxiliary used comprises block copolymers having at least one diene block B and at least one block S comprising a copolymer of a vinylaromatic monomer and 1,1-diphenylethylene 1,1-diphenylethylene substituted by alkyl groups of up to 22 carbons.
 2. A process as defined in claim 1, wherein the dispersing auxiliary used comprises block copolymers having at least one diene block B and at least one block S comprising a copolymer is of a vinylaromatic monomer and 1,1- or its aromatic ring-substituted derivatives, including those substituted by alkyls of up to 22 carbons.
 3. A process as defined in claim 2, wherein the block copolymer comprises polybutadiene or polyisoprene in copolymerized form and the diene block B is partially or completely hydrogenated or unhydrogenated.
 4. A process as defined in either of claims 2 and 3, wherein the block S of the block copolymer consists of a copolymer of styrene and 1,1-diphenylethylene.
 5. A process as defined in any of claims 1 to 4, wherein the dispersing auxiliary is used in an amount of from 0.1 to 10% by weight, based on the amount of vinylaromatic compound employed.
 6. A process as defined in any of claims 1 to 5, wherein aliphatic hydrocarbons are used as dispersion medium.
 7. A process as defined in any of claims 1 to 6, wherein a branching monomer unit comprising at least two vinylaromatic radicals is used in a molar ratio of vinylaromatic monomers to branching units of from 10,000,000:1 to 10:1.
 8. A process as defined in any of claims 1 to 7, wherein the catalyst component A) employed is a metallocene complex of the formula (III)

where R⁷ to R¹¹ are hydrogen, C₁-C₁₀-alkyl, 5- to 7-membered cycloalkyl which in turn can carry C₁-C₆-alkyls as substituents, C₆-C₁₅-aryl or arylalkyl, and where two adjacent radicals may if desired together be cyclic groups of 4 to 15 carbons, or are Si(R¹²)₃, where R¹² is C₁-C₁₀-alkyl, C₆-C₁₅-aryl or C₃-C₁₀-cycloalkyl, M is a metal from subgroups III to VI of the Periodic Table of the elements or is a metal of the lanthanide series, Z¹ to Z⁵ are hydrogen, halogen, C₁-C₁₀-alkyl, C₆-C₁₅-aryl, C₁-C₁₀-alkoxy or C₁-C₁₅-aryloxy and z₁ to z₅ are 0, 1, 2, 3, 4 or 5, the sum z₁+z₂+z₃+z₄+z₅ corresponding to the valency of M minus
 1. 9. A process as defined in claims 1 to 8, wherein the cation-forming compound B) employed comprises open-chain or cyclic alumoxane compounds of the formula IV or V

where R¹³ is C₁-C₄-alkyl and m is an integer from 5 to
 30. 10. A process as defined in any of claims 1 to 8, wherein the cation-forming compound B) employed is a coordination complex compound selected from the group consisting of strong, neutral Lewis acids, ionic compounds having Lewis-acid cations and ionic compounds having Brönsted acids as cations. 